The present invention relates in general to plastic metal working and more specifically to methods for producing billets from low-plastic and hard-to-work materials, predominantly from nickel-, titanium-, and iron-base high-temperature alloys.
The aforementioned alloys find widespread use in modern constructions of power plants and in aerospace engineering. Although said alloys have high temperature strength and resistance to gas corrosion, they are poorly processable due to low plasticity and high strain resistance. This in turn involves high labor-, power-, and material consumption of processes for producing parts from said alloys using metal-working techniques. Special difficulties are encountered in producing billets from superalloys for large-diameter intricate-configuration parts.
A method for processing billets is widely known heretofore as Gatorizing(trademark) (U.S. Pat. No. 3,519,503, 1970). Hard-to-work alloys are processed, according to said method, in two steps. At the first step a fine-grained microstructure is established in the intermediate product by heating the billet to a temperature somewhat lower than the temperature of normal recrystallization and intense plastic deformation involving the ratio of a reduction of cross-sectional area not less than 4:1, press-forming being the predominant deformation technique used for the purpose. However, said technique requires use of high-power pressing equipment due to high straining force involved. At the second step the intermediate product having a fine-grained microstructure undergoes die-forging under superplasticity conditions. Finally, the billet is subjected to finish heat-treatment with a view to restoring its temperature strength. However, the Gatorizing(trademark) process fails to establish a specified microstructure of the billet material.
On the other hand, quite a number of parts from nickel-base superalloys, such as integral rotors, operate under complicated working conditions, whereby it is expedient that special inhomogeneous states of microstructure be established in the various zones of such parts so as to provide an optimum set of properties meeting the actual working conditions of such parts. This, however, is unattainable with the Gatorizing(trademark) process.
One prior-art differential heat-treatment is known (U.S. Pat. No. 3,741,182, 1973), wherein the blade assembly of an integral rotor is subjected to high-heat treatment to form a coarse-grained microstructure therein, whereas the disk of the rotor retains its fine-grained microstructure. The results are that the finished integral rotor has mechanical properties approximating the optimum ones.
The present invention has for its object to provide a method for processing high-temperature alloys, instrumental in establishing specified microstructures in billets of machine parts, both cross-sectionally homogeneous and inhomogeneous, ensuring high technological plasticity when subjected to plastic working, as well as optimum performance characteristics in finished machine components.
The authors of the present invention have discovered quite unexpectedly that the aforesaid object can be accomplished by combining multistep heat-treatment of a billet under certain appropriate temperature conditions and its plastic deformation. Such a processing will hereinafter be referred to as thermomechanical processing.
According to the present invention, thermomechanical processing includes the following steps: heating the billet to a temperature at which a total content of precipitated phases or an allotropic modification of the alloy matrix exceeds 7%, followed by a stepwise decreasing of the process temperature down to a temperature at which a stable fine-grained microstructure is obtained, wherein the ratio between the grain sizes of different phases is not in excess of 10, and billet reduction at the first and each of the following steps of temperature decreasing, with a degree of the billet reduction at each step being a multiple of 1.2 to 3.9 times the change in the billet cross-sectional area.
The foregoing object is accomplished due to preparing the microstructure of billets made from high-temperature nickel-base alloys using the same thermomechanical processing, by a stage-by-stage reduction of the billet processing temperature so as to provide a maximum 14% gain in the xcex3-phase at each stage, and performing post-deformation annealing at the end of each stage of the thermomechanical processing at a temperature not exceeding that at the beginning of the deformation process at the preceding stage.
Of importance for accomplishing the object of the present invention is also the billet strain rate which at the first stage is expedient to be 10xe2x88x922 to 10xe2x88x923 sxe2x88x921, and the following stages be changed in accordance with the following relationship:
xcex5xcex7=Kxcfx86xc2x7xcex5xcex7xcfx81xc2x7Td/Txcex7xcfx81xcfx86
where
xcex5xcex7xe2x80x94strain rate at a next stage;
xcex5xcex7xcfx81xe2x80x94strain rate at a preceding stage;
Tdxe2x80x94deformation temperature;
Txcex7xcfx81xcfx86xe2x80x94temperature of the second phase complete dissolution;
Kxcfx86xe2x80x94empirical coefficient depending on the chemical and phase composition of the alloy (Kxcfx86=0.5-2).
High alloys with predominantly a cast structure, as well as low alloys, are subjected, according to the present invention, to a preliminary thermomechanical processing in a temperature range from 0.95 m.p. of the alloy to a temperature at which the secondary phase content of the alloy is not above 7%, where the processing is carried out with a stepwise temperature decrease, while controlling the alloy temperature and processing temperature and the strain rate at each stage.
Prior to the billet reduction step, it is expedient to place the billet in a heat-insulating container.
One of the specific features of the present invention is the fact that formation of a specified microstructure of the material continues also at the step of subsequent plastic deformation aimed at imparting to the billet the shape of a future finished part by, e.g., rolling said billet.
Prior to said plastic deformation, according to the present invention, the billet is subjected to additional annealing in a monophase region at a temperature not above 1.07 the temperature of the xcex3-phase complete dissolution, followed by cooling at a rate ensuring a gain in the xcex3-phase from about 5% per hour to about 50% per hour, and said additional deformation is carried out at a temperature below the temperature of the xcex3-phase""s complete dissolution. It is expedient that said additional billet annealing be performed in at least two adjacent billet portions so as to establish a temperature gradient there between, the temperature being changed in the range from 0.8 the temperature of complete dissolution of the xcex3-phase in one billet portion to a temperature not above 1.07 the temperature of complete dissolution of the xcex3-phase in the other billet portion.
Said additional billet deformation is carried out after a local shaping pattern occurs in two steps. At the first step the billet is subjected to deformation in a temperature range of superplasticity until the billet size is equal to about 0.6-0.9 of the part""s final size, and at the second step the billet undergoes further deformation until the final part size is obtained, said step being preceded by annealing the billet in a monophase region.
Additional deformation (after a local shaping pattern) in at least two adjoining billet portions is expedient to be carried out with different degrees of reduction varying steadily from one billet portion to another by about 0.25 to 0.75 the degree of reduction of the adjacent billet portion.